(1) Field of the Invention
The invention relates to an on-chip fuse device, and, more particularly, to an electrically programmable, polysilicon fuse with multiple level resistance.
(2) Description of the Prior Art
In the art of integrated circuits, many methods are used to provide manufacturing flexibility. Of particular importance are techniques that allow a common circuit design to be easily configured to specific applications or to parametric requirements with little or no effect on the manufacturing process. An example of a configuration method is the use of on-chip programmable fuses. A fuse is a conductive element that can be made into an open circuit. When the element is a conductor, the circuit operates according to a first configuration. When the element is open circuit, the circuit operates according to a second configuration. A typical prior art, on-chip fuse is simply a metal line. A metal line can be made an open circuit by cutting through the line using a laser. This type of fuse can be very useful. However, it requires a laser cutting source that is external to the circuit.
An alternative method to form a programmable fuse is a polysilicon line. A polysilicon line can be made an open circuit, or blown, by conducting a current through the polysilicon line that exceeds the current density capability of the material. Programmable polysilicon fuses such as this are found in the art. These fuses have a limitation, however, of only being binary devices. The fuses either are a conductive line or are an open circuit. These states may be thought of as a ‘0’ state or a ‘1’ state. It is highly advantageous to form a programmable fuse device that is capable of multiple states.
Several prior art inventions relate to on-chip fuses. U.S. Pat. No. 6,356,496 B1 to Carroll et al describes a method to form polysilicon resistors. The resistors are fuses that can be blown to adjust the value. U.S. Pat. No. 6,242,790 B1 to Tsui et al describes a method to form a variable resistance based on polysilicon resistor/fuses. U.S. Pat. No. 6,175,261 B1 to Sundaraman et al discloses an on-chip fuse circuit.